Functional frequency testing of integrated circuits

ABSTRACT

A method and circuits for testing an integrated circuit at functional clock frequency by providing a test controller generating control signals that assure proper latching of test patterns in scan chains at tester frequency and propagation of the test pattern through logic circuits being tested at functional clock frequency.

This Application is a division of application Ser. No. 10/711,075 filedon Aug. 20, 2004, now U.S. Pat. No. 7,290,191 issued Oct. 30, 2007.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuittesting; more specifically, it relates to a circuit and method fortesting integrated circuits at functional frequency.

BACKGROUND OF THE INVENTION

Integrated circuits, especially logic circuits are often tested usingscan chain methodology, wherein, test patterns are loaded into a set ofscan-in latches, clocked through the combinational logic to be testedand the result pattern captured by scan-out latches for analysis. Thistesting has been traditionally performed at tester frequencies of about30 to 100 MHz. However, with the advent of higher functional frequencyintegrated circuits, for example in about the 1 to 5 GHz range, circuitshave been found to pass at tester frequency but fail at functionalfrequency. Therefore, there is a need for a method and circuit fortesting integrated circuits at functional frequency.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a control circuit, an outputof a first latch connected to an input of a second latch, an output ofthe second latch connected to an input of a third latch, the secondlatch having a feedback connection to an input of the first latch andthe third latch having feedback connections to the first and the secondlatches; combinational logic coupled to the first, second and thirdlatches, the combinational logic having a test signal input, a testclock input and a functional clock input; the feedback connection of thesecond latch further coupled through the combinational logic to a firstcontrol signal output; and the first latch coupled through thecombinational logic to a second control signal output.

A second aspect of the present invention is a method of generatingcontrol signals, comprising: connecting an output of a first latch to aninput of a second latch; connecting an output of the second latch to aninput of a third latch; connecting the second latch through a feedbackconnection to an input of the first latch; connecting the third latchthrough feedback connections to the first and the second latches;coupling combinational logic to the first, second and third latches, thecombinational logic having a test signal input, a test clock input and afunctional clock input; coupling the feedback connection of the secondlatch through the combinational logic to a first control signal output;and coupling the first latch through the combinational logic to a secondcontrol signal output.

A third aspect of the present invention is a control circuit,comprising: an output of a first latch connected to an input of a secondlatch, an output of the second latch connected to an input of a thirdlatch, the second latch having a feedback connection to an input of thefirst latch and the third latch having feedback connections to the firstand the second latches; the combinational logic coupled to the first,second and third latches, the combinational logic having a test signalinput, a scan enable input and a functional clock input; and thefeedback connection of the second latch further coupled through thecombinational logic to a scan control output.

A fourth aspect of the present invention a method of generating acontrol signal, comprising: connecting an output of a first latch to aninput of a second latch; connecting an output of the second latch to aninput of a third latch; connecting the second latch through a feedbackconnection to an input of the first latch; connecting the third latchthrough feedback connections to the first and the second latches;coupling combinational logic to the first, second and third latches, thecombinational logic having a test signal input, a scan enable input anda functional clock input; and coupling the feedback connection of thesecond latch through the combinational logic to a scan control output.

A fifth aspect of the present invention is an integrated circuit, a testpin, a first test clock pin, a second test clock pin, a third test clockpin a functional clock pin, a scan-in pin, a scan-out pin and an enablepin; a test controller having a test input connected to the test pin, afirst test clock input connected to the first test clock pin, afunctional clock input connected to the functional clock pin, a firstcontrol output and a second control output; a clock splitter having afirst clock input connected to the second test clock pin, a second clockinput connected to the functional clock pin, a first control inputconnected to the first control output of the test controller, a secondcontrol input connected to the second control output and of thecontroller, an enable input connected to the enable pin, a ZB clockoutput and a ZC clock output; and an LSSD scan chain comprised ofserially connected latches, a first stage of each latch having a firstdata input, a second data input and a C clock input connected to the ZCclock output of the clock splitter, an A CLK input connected to thethird test clock pin, a second stage of each latch having a data outputand a B clock input connected to the ZB clock output of the clocksplitter, a data output of a previous latch connected to a first inputpin of an immediately subsequent latch, a first data input of a firstlatch of the LSSD scan chain connected to the scan-in pin and a dataoutput pin of a last scan chain latch of the scan chain connected to thescan-out pin.

A sixth aspect of the present invention is a method of testing anintegrated circuit, comprising: providing a test pin, a first test clockpin, a second test clock pin, a third clock pin, a functional clock pin,a scan-in pin, a scan-out pin and an enable pin; providing a testcontroller having a test input connected to the test pin, a first testclock input connected to the first test clock pin, a functional clockinput connected to the functional clock pin, a first control output anda second control output; providing a clock splitter having a first clockinput connected to the second test clock pin, a second clock inputconnected to the functional clock pin, a first control input connectedto the first control output of the test controller, a second controlinput connected to the second control output and of the controller, anenable input connected to the enable pin, a ZB clock output and a ZCclock output; and providing an LSSD scan chain comprised of seriallyconnected latches, a first stage of each latch having a first datainput, a second data input and a C clock input connected to the ZC clockoutput of the clock splitter, an A clock input connected to the thirdtest clock pin, a second stage of each latch having a data output and aB clock input connected to the ZB clock output of the clock splitter, adata output of a previous latch connected to a first input pin of animmediately subsequent latch, a first data input of a first latch of theLSSD scan chain connected to the scan-in pin and a data output pin of alast scan chain latch of the scan chain connected to the scan-out pin.

A seventh aspect of the present invention is an integrated circuit,comprising: a test pin, a select enable pin, a functional clock pin, ascan-in pin and a scan-out pin; a test controller having a test inputconnected to the test pin, a functional clock input connected to thefunctional clock pin, a first control output and a second controloutput; a scan chain comprised of serially connected latches andcorresponding multiplexers, a first stage of each latch having an datainput, a clock input connected to a functional clock pin, a firstcontrol input connected to the first control output of the testcontroller, a second stage of each latch having a data output and asecond control input connected to the second control output of thetester controller, a data output of a previous latch connected to afirst selectable input of a multiplexer corresponding to an immediatelysubsequent latch, a selected output of the corresponding multiplexerconnected to the data input of the immediately subsequent latch, a firstselectable data input of a multiplexer of the scan chain connected tothe scan-in pin and a data output of a last latch of the scan chainconnected to the scan-out pin.

An eighth aspect of the present invention is a method of testing anintegrated circuit, comprising: providing a test pin, a select enablepin, a functional clock pin, a scan-in pin and a scan-out pin; providinga test controller having a test input connected to the test pin, afunctional clock input connected to the functional clock pin, a firstcontrol output and a second control output; providing a scan chaincomprised of serially connected latches and corresponding multiplexers,a first stage of each latch having an data input, a clock inputconnected to a functional clock pin, a first control input connected tothe first control output of the test controller, a second stage of eachlatch having a data output and a second control input connected to thesecond control output of the tester controller, a data output of aprevious latch connected to a first selectable input of a multiplexercorresponding to an immediately subsequent latch, a selected output ofthe corresponding multiplexer connected to the data input of theimmediately subsequent latch, a first selectable data input of amultiplexer of the scan chain connected to the scan-in pin and a dataoutput of a last latch of the scan chain connected to the scan-out pin.

A ninth aspect of the present invention is circuit for testing anintegrated circuit, comprising: a test pin, a select enable pin, afunctional clock pin, a scan-in pin and a scan-out pin; a testcontroller having a test input connected to the test pin, a selectenable input connected to the select enable pin, a functional clockinput connected to the functional clock pin, and a control output; ascan chain comprised of serially connected latches and correspondingde-multiplexers, a first stage of each latch having an data input and aclock input connected to a functional clock pin, a second stage of eachlatch having a data output, a data output of a previous latch connectedto a first selectable input of a multiplexer corresponding to animmediately subsequent latch, a selected output of the correspondingmultiplexer connected to the data input of the immediately subsequentlatch, a first selectable data input of a multiplexer of the scan chainconnected to the scan-in pin and a selected output of a last latch ofthe scan chain connected to the scan-out pin and each multiplexer of thescan chain having a select input connected to the control output of thetest controller.

A tenth aspect of the present invention is a method of testing anintegrated circuit, comprising: providing a test pin, a select enablepin, a functional clock pin, a scan-in pin and a scan-out pin; providinga test controller having a test input connected to the test pin, aselect enable input connected to the select enable pin, a functionalclock input connected to the functional clock pin, and a control output;providing a scan chain comprised of serially connected latches andcorresponding multiplexers, a first stage of each latch having an datainput and a clock input connected to a functional clock pin, a secondstage of each latch having a data output, a data output of a previouslatch connected to a first selectable input of a multiplexercorresponding to an immediately subsequent latch, a selected output ofthe corresponding multiplexer connected to the data input of theimmediately subsequent latch, a first selectable data input of amultiplexer of the scan chain connected to the scan-in pin and aselected output of a last latch of the scan chain connected to thescan-out pin and each multiplexer of the scan chain having a selectinput connected to the control output of the test controller.

BRIEF DESCRIPTION OF DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an integrated circuit test circuitaccording to a first embodiment of the present invention;

FIG. 2 is a schematic circuit diagram of an exemplary clock splitter ofFIG. 1;

FIG. 3 is a schematic circuit diagram of a test controller of FIG. 1;

FIG. 4 is a timing diagram of the integrated test circuit of FIG. 1;

FIG. 5 is a schematic diagram of an integrated circuit test circuitaccording to a second embodiment of the present invention;

FIG. 6 is a schematic circuit diagram of a test controller of FIG. 5;

FIG. 7 is a timing diagram of the integrated test circuit of FIG. 5;

FIG. 8 is a schematic circuit diagram of an exemplary implementation ofan master/slave scan latch of FIG. 5;

FIG. 9 is a schematic diagram of an integrated circuit test circuitaccording to a third embodiment of the present invention;

FIG. 10 is a schematic circuit diagram of a test controller of FIG. 9

FIG. 11 is a schematic circuit diagram of an exemplary implementation ofan master/slave scan latch of FIG. 9;

FIG. 12 is a timing diagram of the integrated test circuit of FIG. 9;

FIG. 13 is a schematic circuit diagram of a first compact clock splitteraccording to the present invention;

FIG. 14 is a timing diagram of the compact clock splitter of FIG. 13;

FIG. 15 is a schematic circuit diagram of a second compact clocksplitter according to the present invention; and

FIG. 16 is a timing diagram of the compact clock splitter of FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

A cycle is defined herein as a transition of a signal from a first stateto a second state, continuance of the signal at said second state for afixed period of time, transition from the second state back to the firststate and continuance of the signal at the first state for the samefixed period of time. A pulse is defined as a transition of a signalfrom a first state to a second state, continuance of the signal at thesecond state for a fixed period of time and transition from the secondstate back to the first state. A pin is defined herein as an integratedcircuit pad, a circuit input or output or a circuit node.

FIG. 1 is a schematic diagram of an integrated circuit test circuitaccording to a first embodiment of the present invention. The firstembodiment of the present invention is an application of the presentinvention to level sensitive scan design (LS SD) testing methodology. InFIG. 1, integrated circuit 100 includes a test controller 105, amultiplicity of clock splitters 110 each supplying a ZC clock signal anda ZB clock signal to LSSD scan chains 115, each scan chain 115 includinga multiplicity of L1L2 scan latches 120. Only one scan chain 115 isillustrated in FIG. 1, but each clock splitter 110 is capable ofsupplying the ZC and ZB clock signals to other scan chains 115.

Test controller 105 includes a functional frequency clock input pin(OSC), a B test clock pin (B), a test mode pin (TEST), a first controlsignal output pin (CN1) and a second control signal output pin (CN2).Test controller receives a functional frequency clock signal OSC (thisis the clock signal used during normal operational mode of theintegrated circuit) at the OSC pin, a test clock B signal at the B pinand a test mode signal TEST at the TEST pin. Test controller generates afirst control signal CN1 at the CN1 pin and a second control signal CN2at the CN2 pin. Test controller 105 is illustrated in FIG. 3 anddescribed infra.

Each clock splitter 110 includes a CN1 input pin (CN1), a CN2 input pin(CN2), a C test clock input pin (C), a functional frequency clock inputpin (OSC), an optional enable signal input pin (EN), a functionalfrequency C clock output pin (ZC) and a functional frequency B clockoutput pin (ZB). Each clock splitter receives from test controller 105,the first control signal CN1 at pin CN1, the second control signal CN2at pin CN2, a TEST CLK C signal at pin C, the OSC signal at pin OSC andthe EN signal at pin EN. Each clock splitter 110 generates atest/functional frequency clock ZC signal at pin ZC and atest/functional frequency clock ZB signal at pin ZB. Clock splitter 110is illustrated in FIG. 2 and described infra.

Each L1L2 scan latch 120 in scan chain 115 includes an L1 section havingan input pin (I), a data pin (D), an A clock signal pin (A) and a Cclock signal pin (C) and an L2 section having a output pin (Q) and a Bclock input pin (B). The D and Q pins are coupled to the combinationallogic (not shown) being tested and all L1L2 scan latches 120 are coupledin series by connecting the Q pin of a previous L1L2 scan latch to the Ipin of the immediately subsequent L1L2 scan latch. Test patterns arescanned in to the I pin of the first L1L2 scan latch 120 in scan chain115 and resultant patterns are scanned out through the Q pin of the lastL1L2 scan latch in the scan chain. The C pin of each L1L2 scan latch 120receives the ZC signal and the B pin of each L1L2 scan latch receivesthe ZB signal from one of the clock splitters 110. L1L2 latches are wellknown in the art. The A pin of each L1L2 scan latch receives a TEST CLKA signal.

A test pattern is loaded into scan chain 115 by a series of TEST CK A-ZBpulses. The TEST CLK A pulse captures in the L1 section data present atthe I pin and the ZB pulse moves the data from the L1 section to the L2section. ZB is at tester frequency (TEST B CLK frequency) during thisoperation. A ZC pulse then moves data at pin D into the L1 section. TheZC pulse may be at tester frequency (TEST CLK C frequency) or functionalfrequency (OSC frequency). When ZC is at functional frequency, controlsignals CN1 and CN2 generated by test controller 105 eliminate theproblem of “sequential depth” (the replacement of the test pattern withthe data in the logic circuits upon switching ZB and ZC from testerfrequency to functional frequency) by ensuring a functional frequency ZBpulse is issued first.

The tester generates TEST CLK A, TEST CLK B and TEST CLK C and in oneexample have frequencies of about 30 to 100 MHz, however these speedsmay be expected to increase as testers become faster. OSC is generatedby the integrated circuit chip itself and in one example has a frequencyof about 1 to 5 GHz, however this speed may be expected to increase asintegrated circuits become faster. Clock signals ZC and ZB are generatedby clock splitters 110 and have the same frequency and the leading andtrailing pulse edges of clock signals ZB and ZC are aligned respectivelyto leading and trailing clock edges of clock signal OSC.

FIG. 2 is a schematic circuit diagram of an exemplary clock splitter 110of FIG. 1. In FIG. 2, clock splitter includes NAND gates N1, N2, N3 andN4 and inverters I1, I2 and I3. A first input of NAND gate N1 is coupledto OSC and a second input of NAND gate N1 is coupled to EN. The outputof NAND gate N1 is coupled to a first input of NAND gate N2 and a secondinput of NAND gate N2 is coupled to TEST CLK C. The output of NAND gateN2 is coupled to a first input of NAND gate N4 and to a first input ofNAND gate N3 through inverter I1. A second input of NAND gate N4 iscoupled to CN2 and a second input of NAND gate N3 is coupled to CN1. Theoutput of NAND gate N3 is coupled to the input of inverter I2, theoutput of inverter I2 being ZC. The output of nand gate N4 is coupled tothe input of inverter I3, the output of inverter I3 being ZB.

When EN, CN1, CN2 and TEST CLK C are high, clock splitter generates ZBand ZC at functional frequency (OSC frequency) and the state of OSCcontrols the state of ZB and ZC. When OSC is high, ZB is high and ZC islow. When OSC is low, ZB is low and ZB is high.

When TEST CLK C is low, Clock splitter output ZC is low and ZB iscontrolled by CN2. When CN2 is low, ZB is low and when CN2 is high, ZBis high.

When TEST CLK C is high and CN2 is low, clock splitter ZB is low and ZCis controlled by CN1, OSC, and EN. When CN1 is high and either OSC islow or EN is low, ZC is high. When CN1 is low or when or when OSC and ENare both high, ZC is low.

FIG. 3 is a schematic circuit diagram of test controller 105 of FIG. 1.In FIG. 3, test controller 105 includes latches Q1, Q2 and Q3, whichform a state machine. Latch Q1 includes a NAND gate N7, an And/Or/Invert(AOI) gate AOI2 and a NOR gate N8. The output of NAND gate N7 isconnected to a first AND input of AOI2 and the output of AOI2 isconnected to a node Q1B that is connected to a first input of NOR gateN8. A first input of NAND gate N7 is connected to a second AND input ofAOI2 and the output of NOR gate N8 is connected to a third AND input ofAOI2.

Latch Q2 includes an inverter 16, a NAND gate N9, and AOI3 and a NORgate N10. The output of NAND gate N9 is connected to a first AND inputof AOI3 and the output of AOI3 is connected to a node Q2B that isconnected to a first input of NOR gate N10. The output of NOR gate N9 isconnected to a second AND input of AOI3. The output of NOR gate N8 isconnected to a third AND input of AOI3. Node Q2B is connected to asecond input of NAND gate N7.

Latch Q3 includes an AOI4 and a NOR gate N11. The output of AOI4 isconnected to a node Q3B that is connected to a first input of NOR gateN11. The output of NOR gate N11 is connected to an OR input of AOI4. Theoutput of NOR gate N10 is connected to a first AND input of AOI4.

A fourth AND input of AOI2 and a fourth AND input of AOI3 are connectedto node Q3B. The first input of NAND gate N7 is connected to a secondAND input of AOI4.

A TEST signal is supplied the input of inverter I5 and the output ofinverter I5 (a signal TESTB) is coupled to a second input of NOR gateN8, a second input of NOR gate N10 and a second input of NOR gate N11.The output of NOR gate N8 is also connected to a first input of NANDgate N9. TESTB is coupled to a first input of a NOR gate N6 and OSC iscoupled to a second input of NOR gate N6. The output of NOR gate N6 isconnected to the input of inverter I6 as well as to the first input ofNAND gate N7 and a second AND input of AOI4. The output of inverter 16is connected to a second input of NAND gate N9 and a fifth AND input ofAOI3.

TEST is also coupled to a first input of a NAND gate N5A and a first ANDinput of AOI1. Node Q1B is connected to a second AND input of AOI1 andnode Q2B is connected to a second input of AND gate N5A. A TEST CLK B iscoupled to an input of an inverter I4 and the output of inverter I4 isconnected to the OR input of AOI1. The output of NAND gate N5A iscoupled to CN1, and the output of AOI1 is coupled to CN2.

A low on input TEST resets latches Q1, Q1, Q2 low in preparation for atest cycle at functional frequency. After TEST goes high, there are 2startup states for Q1. If OSC was stopped low, Q1 will go high and ifTEST CLK B is high, turn CN2 high in anticipation of positive functionalclock pulses. If OSC was stopped high, a low in latch Q1 holds CN2 lowuntil the first time OSC goes low setting Q1 high, allowing CN2 to gohigh if TEST CLK B is high. With TEST CLK B is high and CN2 high thecircuit is ready to pass the first OSC pulse. When CN2 is high, then theSlave latch of the flip-flop can be updated.

At this time latch Q2 will remain low, holding CN1 low. This preventsthe master latch of the flip-flops from being updated until the nexttime OSC goes high after Q1 went high. When OSC goes high after Q1 goinghigh, Q2 will go high driving CN1 high. This then allows the masterlatch of the flip-flops to be updated.

Latch Q3 is low until the next time OSC goes high after Q2 went high.Latch Q3 starts the termination of the control sequence and will remainhigh insuring that the sequence does not restart until reset by the TESTsignal going low.

Latch Q2 remains high holding CN1 high until OSC goes high after whichQ2 goes low. The Master latch has been updated and Q2 low preventsfuture updates.

Latch Q1 is high holding CN2 high until OSC goes low after Q2 has gonelow. At this time Q1 goes low. This stops the clock sequence and nofurther changes to the flip-flop data in either the master or slavelatches can occur until the circuit has been reset by TEST going low.

For the following discussion of the operation of test controller 105,reference to the timing diagram of FIG. 4 as well as FIGS. 1 through 3will be helpful. Test controller 105 is off when TEST is low causingboth CN1 and CN2 to remain high and integrated circuit 125 is inoperational mode and there are no delay or switching impacts to clocksplitters 110. Test controller 105 is in test mode when TEST is high andTEST CLK B is held high, forcing both CN1 and CN2 low. In test modethere is no change in CN1 or CN2 unless OSC is low or until the firstfalling edge of OSC. On the first falling edge of OSC, CN2 goes high. Onthe first rising edge of OSC, CN1 goes high. On the next rising edge ofOSC CN1 goes low. On the next falling edge of OSC, CN2 goes low.Thereafter CN1 and CN2 remain low regardless of the state of OSC. ThusCN2 is high for two OSC cycles and CN1 is high for one OSC cycle, butCN1 and CN2 are a half an OSC cycle out of phase. The result of clocksplitter 110 (see FIG. 1) receiving these timed and coordinated CN1 andCN2 signals is to cause the clock splitter to generate a ZB high pulsefollowed by a ZC high pulse followed by a ZB high pulse, the ZC beinghigh between ZB pulses with the ZB and ZC pulses at the OSC frequencyand being a half cycle out of phase (see FIG. 4). The first ZB pulsehigh causes data in the L1 section of the L1L2 scan latches 120 (seeFIG. 1) to be transferred (or launched) from the L1 section to the L2section and into the combinatorial logic connected to the output Q ofthe L2 section. The ZC pulse high causes data on the D pin to becaptured in the L1 section. The second ZB high pulse again moves data inthe L1 section to the L2 section but with no further ZB or ZC pulses nofurther data transfer is possible. Thus, the test vector is flushed outof L1L2 scan latches 120 and replaced with the response to thecombination logic being tested.

Because CN2 is a half OSC cycle ahead of ZB and ZC is a half OSC cycleahead of ZC, enablement of the data capture of L1 stages and transferbetween L1 and L2 stages is enabled and disabled a half an OSC cycleahead of time thus relaxing timing constraints of integrated circuit 100(see FIG. 1). Further, because further OSC pulses are ignored by testcontroller 105 (see FIG. 1), OSC can continue to run and circuitsdownstream on the clock tree will not be effected.

Test controller 105 (see FIG. 1) can be easily modified to respond to anegative clock design (i.e. where a falling OSC edge results in datatransfer and a rising edge results in data capture). Test controller 105can be further modified to provide additional transfer and/or, capturesequences by adding additional QX latch stages.

FIG. 4 is a timing diagram of the integrated test circuit of FIG. 1. Thetimings of TEST CLK B, TEST CLCK C, OSC, TEST, CN1 CN2 have beendiscussed supra. Q3B represents the timing of the signal on node Q3B(see FIG. 3) during operation of test controller 100 (see FIG. 3).

FIG. 5 is a schematic diagram of an integrated circuit test circuitaccording to a second embodiment of the present invention. The secondembodiment of the present invention is an application of the presentinvention to MUX Scan Latch testing methodology. In FIG. 5, integratedcircuit 125 includes test controller 185, and a scan chain 130. As thisis a Mux Scan design, a separate TEST CLK B is not needed. Scan chain130 includes a multiplicity of multiplexers 135 and a correspondingmultiplicity of master/slave flip flop (MS) scan latches 140. Each MSscan latch 140 in scan chain 130 includes a master (M) section having adata pin (D), CLK clock signal pin (CLK) and first control pin (CN1) anda slave (S) section having a output pin (Q) and a second control pin(CN2). MS scan latches 140 are illustrated in FIG. 8 and describedinfra. Each multiplexer 135 in scan chain 130 includes an input pin (I),a data pin (D) a select enable pin (SE) which selects whether pin I orpin D is coupled to the output of the de-multiplexer based on an SEsignal from the tester.

The D pin of each multiplexer 135 and the Q pin of each MS scan latchare coupled to the combinational logic (not shown) being tested. Theoutput of each multiplexer 135 is connected to the D pin of acorresponding MS scan latch 140 and all MS scan latches are coupled inseries by connecting the Q pin of a previous MS scan latch to the I pinof a multiplexer 135 connected to an immediately subsequent MS scanlatch. The CLK pin of each MS scan latch 140 is connected to thefunctional clock pin. The CN1 pin of each MS scan latch 140 is connectedto the CN1 of test controller 105 and the CN2 pin of each MS scan latchis connected to the CN2 pin of the test controller.

While MS latches are well known in the art, MS scan latches 140 aremodified to accept and function with the CN1 and CN2 signals. MS scanlatches 140 are illustrated in FIG. 8 and described infra. The CLK pinof each MS scan latch receives OSC. The present MS scan latches 140generate an OSC BAR (the inverse of OSC) internally, the S section beingresponsive to OSC and the M section being responsive to OSC BAR. Aseparate OSC BAR may be supplied.

Test patterns are scanned in to the I pin of the first multiplexer 135in scan chain 130 and resultant patterns are scanned out through the Qpin of the last MS scan latch 140 in the scan chain. A test pattern isscanned into scan chain 130 by setting SE to select I inputs and TESTlow (off), and OSC is cycled to load the test pattern into scan chain130.

The operation of integrated circuit 125 is similar to that of integratedcircuit 100 of FIG. 1. Test controller 185 is off when TEST is lowcausing both CN1 and CN2 to remain high. Integrated circuit 125 is inoperation mode when TEST is low. In operational mode when SE is heldlow, multiplexers 135 select D. MS scan latches 140 operate normallycapturing the state at the M section of MS scan latch 140 and allowingthe S section of MS scan latch 140 to sample the M section and presentthe state of the M section at the output of the latch on the rising edgeof OSC. In scan mode when SE is high, multiplexers 135 select I and MSscan latch 140 functions as described for normal operation except thatthe input for the M section is now from I. Integrated circuit 125 issequenced into test mode by holding SE high while bringing TEST high,forcing both CN1 and CN2 low (CN2 may remain high if OSC stopped low).This preserves the scan state of the master and S sections of latch 140and then SE is brought low causing multiplexers 135 to select D. In testmode there is no change in CN1 or CN2 unless OSC is low or until thefirst falling edge of OSC. On the first falling edge of OSC, CN2 goeshigh. This enables the slave portion of MS scan latch 140 to be updatedon the next rising edge of OSC. CN1 is low at this time preventing the Msection of MS scan latch 140 from sampling D and preserving the scan incontents. On the first rising edge of OSC, the slave portion of MS scanlatch 140 samples the state of the master portion of MS scan latch 140and presents the data to the downstream logic. CN1 also goes highenabling the M section of MS scan latch 140 to sample the logic response(D) when OSC next goes low. On the next rising edge of OSC, the Ssection of MS scan latch 140 now samples the state of the M section ofMS scan latch 140 and presents this state to the downstream logic. Alsoat this time CN1 goes low, this prevents the M section of MS scan latch140 from changing due to the new data presented to the logic. On thenext falling edge of OSC, CN2 goes low and prevents the slave portion ofMS scan latch 140 from changing. Thereafter CN1 and CN2 remain low andthe master and S sections of MS scan latch 140 contain the response ofthe logic to the scanned in test vector regardless of the state of OSC.Thus CN2 is high for two OSC cycles and CN1 is high for one OSC cycle,but CN1 and CN2 are a half an OSC cycle out of phase. CN2 going highcauses data in the M section of MS scan latch 140 to be transferred (orlaunched) from the M section to the S section and into the combinatoriallogic connected to the S section output on the next rising edge of OSC.Again, this enablement is a half an OSC clock cycle early. On a risingedge of OSC, CN1 going high causes data on the Q pin of the previous MSscan latch 140 is captured in the M section of the subsequent MS scanlatch. Again, this enablement is a half an OSC clock cycle early. On thenext rising edge of OSC, CN1 goes low and stays low. Again, thisdisablement is a half an OSC cycle early. On the next OSC falling edge,CN2 goes low and stays low. Again, this disablement is a half an OSCcycle early. Because CN2 changes on the falling edge of OSC and CN1changes on the rising edge of OSC, enablement of the data capture of Mstages and transfer between M and S stages is enabled and disabled ahalf an OSC cycle ahead of time thus relaxing timing constraints ofintegrated circuit 125.

Further, because further OSC pulses are ignored by test controller 185(see FIGS. 5 and 6), OSC can continue to run and circuits downstream onthe clock tree will not be affected.

FIG. 6 is a schematic circuit diagram of test controller 185 of FIG. 5.Test controller 185 is similar to test controller 105 of FIG. 3 exceptthe TEST CLK B pin (there is no longer a TEST CLK B), inverter 14 andAOI1 of FIG. 3 are replaced with a NAND gate N5B.

FIG. 7 is a timing diagram of the integrated test circuit 125 of FIG. 5.The timings of OSC, SE, TEST, Q3B, CN1 CN2 have been discussed supra.Integrated circuit 125 is sequenced from scan mode to test mode and thenback scan mode as shown.

Test controller 185 (see FIG. 6) can be easily modified to respond to anegative clock design (i.e. where a falling OSC edge results in datatransfer and a rising edge results in data capture). Test controller 185(see FIG. 6) can be further modified to provide additional transferand/or, capture sequences by adding additional QX latch stages.

FIG. 8 is a schematic circuit diagram of an exemplary implementation ofMS scan latch 140 of FIG. 5. Other implementations using a differentbase latch design and adding CN1 and CN2 inputs are possible. In FIG. 8,MS scan latch 140 includes an M section 145A and an S section 145B, ANDgates 150A and 150B and inverters 155A, 155B and 155C. M section 145Aincludes PFETs T1, T2, T5, T6 and T9 and NFETs T3, T4, T7, T8 and T10and an output node MS. S section 145B includes PFETs T11, T12, T15, T16,T19 and T21 and NFETs T13, T14, T17, T18, T20 and T22. AND gate 150Aincludes PFETs T23 T26 and NFETs T24, T25 and T27 and an output node C1.AND gate 150B includes PFETs T28, T31 and NFETs T29, T30 and T32 and anoutput node C2. Inverter 155A includes a PFET T33 and an NFET T34 and anoutput node C3. Inverter 155B includes a PFET T35 and an NFET T36 and anoutput node C4. Inverter 155C includes a PFET T37 and an NFET T38 and anoutput node C5.

The D input pin of MS scan chain latch 140 is connected to the gates ofPFET T1 and NFET T4 of M section 145A, and the Q output pin of MS scanchain latch 140 is connected to the drains of PFET T21 and NFET T22 of Ssection 145B. The CLK pin of MS scan latch 140 is connected to thedrains PFET T23 and NFET T24 of AND gate 150A. The CN2 pin of MS scanchain latch 140 is connected to the gates of PFET T26 and NFETs T24 andT27 of AND gate 150A. Output node C1 of AND gate 150A is connected tothe gates of PFET T33 and NFET T34 (the input) of inverter 155A. Thedrains of PFET T33 and NFET T34 (output node C3) of inverter 155A areconnected to the drains of PFET T28 and NFET T29 of AND gate 150B. TheCN1 pin of MS scan chain latch 140 is connected to the gates of PFET T31and NFETs T29 and T32 of AND gate 150B. Output node C2 of AND gate 150Bis connected to the gates of PFET T35 and NFET T36 (the input) ofinverter 155B. The drains of PFET T35 and NFET T36 (output node C4) areconnected to the gates of PFET T37 and NFET T38 (the input) of inverter155C. The drains of PFET T37 and NFET T38 (output node C5) of inverter155C are connected to the gates of NFET T3 and PFET T6 of M section145A. Output node C4 of inverter 155B is connected to the gates of PFETT2 and NFET T7 of M section 145A. Output node C3 of inverter 155A isconnected to the gates of PFET T12 and NFET T17 of S section 145B.Output node C1 of AND gate 150A is connected to the gates of PFET T16and NFET T13 of S section 145B.

AND gate 150B allows CN1 to control by latching of data on the D pin byM section 145A by gating CLK. AND gate 150A allows CN2 to control bytransfer of data in M section 145A into S section 145B by gating CLK.

FIG. 9 is a schematic diagram of an integrated circuit test circuitaccording to a third embodiment of the present invention. The thirdembodiment of the present invention is also an application of thepresent invention to MUX Scan Latch testing methodology, but usingunmodified or standard MS latches. In FIG. 9, integrated circuit 160includes test controller 165, and a scan chain 170. Scan chain 170includes a multiplicity of multiplexers 175 and a correspondingmultiplicity of MS scan latches 180. Each MS scan latch 180 in scanchain 170 includes an M section having a data pin (D) and a clock signalpin (CLK) and an S section having a output pin (Q). Each multiplexer 175in scan chain 170 includes an input pin (I), a data pin (D) a selectenable pin (SE) which selects whether pin I or pin D is connected to theoutput of the de-multiplexer. MS scan latches 180 are illustrated inFIG. 11 and described infra. Test controller 165 includes an OSC inputpin, a TEST input pin (for respectively receiving the OSC and TESTsignals described supra) a select enable input pin for receiving the SEsignal (described supra) from the tester, and a CNSE output pin for aCNSE signal that controls the select function of each multiplexer 175.Test controller 165 is illustrated in FIG. 10 and described infra.

The D pin of each de-multiplexer 175 and the Q pin of each MS scan latchare coupled to the combinational logic (not shown) being tested. Theoutput of each multiplexer 175 is coupled to the D pin of acorresponding MS scan latch 180 and all MS scan latches are coupled inseries by connecting the Q pin of a previous MS scan latch to the I pinof a multiplexer 175 connected to an immediately subsequent MS scanlatch. The SE pin of each de-multiplexer 175 is connected to the CNSEpin of test controller 165 and each CLK pin each MS scan latch 180 iscoupled to the functional clock pin. The present MS scan latches 140generate CLK BAR (the inverse of CLK) internally, the S section beingresponsive to CLK and the M section being responsive to CLK BAR. Aseparate CLK BAR may be supplied.

Test patterns are scanned in to the I pin of the first de-multiplexer175 in scan chain 170 and resultant patterns are scanned out through theQ pin of the last MS scan latch 180 in the scan chain. A test pattern isscanned into scan chain 170 by setting TEST to low (off) and SE toselect the I inputs of de-multiplexers 175 and the test pattern intoscan chain 170 as OSC cycles.

FIG. 10 is a schematic circuit diagram of test controller 165 of FIG. 9.Tester control 165 is similar to tester controller 105 except AOI1 andNAND gate N5A are replaced with AND gate A12 and NOR gate N13, TEST CLKB is replaced with SE, CN1 and CN2 are replaced with CNSE. TEST iscoupled to a first input of AND gate N12 and node Q2B is coupled to asecond input of AND gate A12. SE is coupled through an inverter I19 to afirst input of NOR gate N13, the output of AND gate A12 (a node N1) isconnected to a second input of NOR gate N13 and the output of NOR gateN13 is connected to a CNSE output pin.

FIG. 11 is a schematic circuit diagram of an exemplary implementation ofMS scan latches 180 of FIG. 9. In FIG. 9, MS scan latch 180 is similarto MS scan latch 140 of FIG. 8, except there are no AND gates 150A and150B or CN1 or CN2 signals, CLK is connected directly to the input ofinverter 155A and node C3 is connected directly to the input of inverter155B.

For the following discussion of the operation of integrated circuit 160,reference to FIGS. 9 through 12 will be helpful. Returning to FIG. 9,during operation of integrated 160, test controller 165 is off when TESTis low causing CNSE to follow the state of SE. If SE is low themultiplexers and integrated circuit 160 are in operational mode andthere is no delay or switching impact to scan chain 170. Test controller165 is in test mode when TEST is high and CNSE now has a more complexlogical function. In the functional speed test mode, SE remains high andthen TEST is pulled high. OSC can be stopped high or low. Then two atspeed clock pulses are propagated down OSC. On a first rising edge ofOSC, the S section of MS scan latch 180 samples the contents of the Msection of MS latch 180 and presents this value to the logic under test.The M section of MS scan latch 180 contains the values that were scannedin for test. CNSE is also pulled low switching the multiplexer 175 frompassing I to passing D to the M section of MS scan latches 180 tosample-the data on their D pins on the next falling edge of OSC. Againthis enablement is a half a cycle early. On the next rising edge of OSC,the S section of MS scan latches 180 samples the data on the m sectionof MS scan latches 180. CNSE also goes high and stays high. Thisprevents any further data captures from the combinational logicoccurring on the M section of MS scan latches 180. Further OSC cyclescan-out the test data, it would be preferred at this point to switchback to scan mode and use the slower speed scan clock. TEST would bepreferably pulled high but this is not required.

FIG. 12 is a timing diagram of the integrated test circuit of FIG. 9.The timing of SE, OSC, TEST and CNSE have been described supra. Node N1is high when TEST is low, goes low when TEST goes high, goes high againwhen CNSE goes low and goes low again (and stays low) when CNSE goeshigh.

The first embodiment of the present invention described supra andillustrated in FIG. 1, utilized a separate test controller and clocksplitter. The fourth and fifth embodiments of the present inventionutilize a combined test controller and clock splitter circuit.

FIG. 13 is a schematic circuit diagram of a first compact clock splitteraccording to the present invention. In FIG. 13, a compact test splitter190 includes a first latch Q4, a second latch Q5, a third latch Q6,(latches Q4, Q5 and Q6 forming a state machine) an output stage CS1,inverters I7 and I12, NOR gate N19, AOI5, a C test clock pin (TEST CLKC), an enable pin (EN), an OSC pin (OSC), a B test clock pin (TEST CLKB), a mode pin (TEST) a first output pin (ZC) and a second output pin(ZB).

First latch Q4 includes NAND gates N20 and N21, inverter I11 andOr/And/Invert gate (OAI) OAI8. The output on NAND gate N20 is connectedto a first input of NAND gate 21, the output of inverter I11 isconnected to a first OR input of OAI8. The output of OAI8 (node Q4B) isconnected to a second input of NAND gate N21 and the output of NAND gateN21 is connected to a first AND input OAI8.

Second latch Q5 includes an OAI6 and an OAI7. The output of OAI6 (nodeQ5B) is connected to an AND input of OAI7 and the output of OAI7 isconnected to the AND input of OAI6.

Third latch Q6 includes NAND gates N22 and N23. The output of NAND gateN22 is connected to a first input of NAND gate N23, and the output ofNAND gate N23 (node Q6B) is connected to a first input of NAND gate N22.

Output stage CS1 includes NAND gates N14, N15, and N18, NOR gates N16and N17 and inverters I9 and I10. TEST CLK C is connected to a firstinput of NAND gate N15. EN is connected to a first input of NAND gateN14. OSC is connected to a second input of NAND gate N14 and a firstinput of NOR gate N19. The output of NAND gate N14 is connected to asecond input of NAND gate N15, and the output of NAND gate N15 isconnected to a first input of NOR gate N17 and a first input of NANDgate N18. The output of NOR gate N16 is connected to a second input ofNOR gate N17. The output of NOR gate N17 is coupled to ZC throughserially connected inverters 18 and 19. The output of NAND gate N18 iscoupled to ZB through inverter I10.

Interconnections between output stage CS1 and latch stages Q4, Q5 and Q6are as follows: TEST CLK B is coupled to the OR input of AOI5 throughinverter 112. The output of AOI5 is connected to a second input of NANDgate N18. TEST is coupled to a second input of NOR gate N19 and a firstinput of NOR gate N16 through inverter I7, and is connected to a secondAND input of OAI8, a first AND input of AOI5 and a second input of NANDgate N23. The output of NAND gate N23 (Q6B) is also connected to asecond AND input of AOI5, a second OR input of OAI8 and a first input ofNAND gate N20 output of NOR gate N19 is connected to a second input ofNAND gate N20, the input of inverter I11, a first OR input of OAI6 and afirst OR input of OAI7. The output of OAI8 (Q4B) is also connected to asecond OR input of OA16 and a third AND input of AOI5. The output ofOAI7 is also connected to a second input of NAND gate N22, and theoutput of OA16 (Q5B) is also connected to a second input of NOR gateN16, and the output of NAND gate 21 is also connected to a second ORinput of OAI17.

The operation of compact clock splitter 190 is best understood withreference to FIGS. 14 and 15. FIG. 14 is a timing diagram of compactclock splitter 190 of FIG. 13. Compact clock splitter 190 takes OSC andgenerates the non-overlapping clock pulses ZB and ZC used to controlLSSD scan chains as described supra. ZB goes high when OSC goes high andallows transfer of data between L1 and L2 latches of the L1/L2 LSSDlatches. ZC goes high when OSC goes low and latches data at the D pinsof the L1 latch of the L1/L2 LSSD latches. The time period from therising edge of ZB to the falling edge of ZC is the time period for datato propagate through the L2 latch of the L1/L2 LSSD latch, through thelogic being tested and then be latched by the L1 latch. The ZB-ZC pulseduration is from the first rising edge of OSC to the next rising edge ofOSC.

When in functional mode, TEST is low. Test low forces the output ofinverter I7 high and the output of NOR gate N16 low, disabling ZCcontrol. As NOR gate N16 output is a DC non-switching level in thismode, no delay is added. TEST low also drives the AND output of AOI5low, removing AOI5 from the ZB control path. TEST low also drives nodesQ4B and Q6B to high states and node Q5B to a low state. The output of I7further forces NOR gate N19 low. This stops OSC from propagating throughnodes Q4B, Q5B and Q6B eliminating switching power in the functional aswell as any test mode where TEST is low.

In test mode, TEST goes high, the output of inverter I7 goes low and NORgate N16 is enabled to control ZC as a function of the state of nodeQ5B, and NOR gate N19 is enabled to allow OSC to propagate to latches Q4and Q5. If OSC is high when TEST goes high, node Q4B will remain highuntil the falling edge of OSC to prevent ZB from going high. As node Q4Bwill be in a low state or go low at a falling edge of OSC, ZB is enabledone half a cycle in advance of the first rising edge of OSC and no delayresults, allowing the ZB pulse to occur with the same timing as OSC infunctional mode.

With the first rising edge of OSC (and node Q4B low), node Q5B goeshigh, returning control of ZC to OSC. As this occurs one half cyclebefore the falling edge of OSC, no delay results, allowing the ZC pulseto occur with the same timing as OSC in functional mode.

Also with the first rising edge of OSC (and node Q4B low), node Q6B goeslow, setting up latch Q4 so node Q4 b will revert high on the nextfalling edge of OSC, and allowing node Q5B to go low on the next risingedge of OSC. As the logic path delay through latch Q5 and NOR gate N16to NOR gate N17 is longer than the delay from OSC through NAND gates N14and N15 to NOR gate N17, the rising edge of OSC terminates the ZC pulseand then the falling edge of the signal at node Q5B arrives later andeliminates the possibility of any further ZC pulses and allowing the ZBpulse to occur with the same timing as OSC in functional mode.

It should be noted that the functional operation of latches Q4, Q5 andQ6 just described is essentially the same as the operation of latchesQ1, Q2 and Q3 of tester controller 105 of FIG. 3 or 165 of FIG. 10described supra, except CN1, CN2 and CNSE are operated upon, not ZB andZC directly.

The pattern of ZB and ZC pulses produced by compact clock splitter 190is always ZB first, ZC second, and ZB third, the rising edge of ZCalways aligned with the falling edge of the first ZB pulse and thefalling edge of the ZC pulse aligned with the rising edge of the secondZB pulse. Thereafter, a series of ZB pulses are produced which cause noLSSD state changes but do consume some switching power. These“un-needed” ZB pulses are eliminated in the fifth embodiment of thepresent invention.

FIG. 15 is a schematic circuit diagram of a second compact clocksplitter according to the present invention. In FIG. 15, a compact testsplitter 195 includes a first latch Q7, a second latch Q8, a third latchQ9 (latches Q7, Q8 and Q9 forming a state machine), an output stage CS2,a NAND gate N37, a C test clock pin (TEST CLK C), an enable pin (EN), anOSC pin (OSC), a B test clock pin (TEST CLK B), a mode pin (TEST) afirst output pin (ZC) and a second output pin (ZB).

First latch Q7 includes NAND gates N28, N29, N30, N31 and N32 and aninverter I16. The output of NAND gate N28 is connected to a first inputof NAND gate N29 and the input of inverter I16. The output of inverterI16 is connected to a first input of NAND gate N30. The output of NANDgate N30 is connected to a first input of NAND gate N31. The output ofNAND gate N29 is connected to a first input of NAND gate N32. The outputof NAND gate N32 is connected to a second input of NAND gate N31 and theoutput of NAND gate N31 (a node Q7B) is connected to a second input ofNAND gate N32.

Second latch Q8 includes NAND gates N33, N34, N35 and N36 and invertersI17 and I18. The output of inverter I18 is connected to a first input ofNAND gate N35 and a first input of NAND gate N33. The second input ofNAND gate N33 is connected to the input of inverter I17 and the outputof inverter I17 is connected to a second input of NAND gate N35 outputof NAND gate N33 is connected to a first input of NAND gate N34 and theoutput of NAND gate N35 is connected to a first input of NAND gate N36.The output of NAND gate N36 is connected to a second input of NAND gateN34 and the output of NAND gate N34 (a node Q8B) is connected to asecond input of NAND gate N36.

Third latch Q9 includes NAND gates N38, N39 and N40. The output of NANDgate N39 is connected to a first input of NAND gate N40. The output ofNAND gate N40 is connected to a first input of NAND gate N38 and theoutput of NAND gate N38 (a node Q9B) is connected to a second input ofNAND gate N40.

Output stage CS2 includes NAND gates N24, N25, N26 and N27 and invertersI13, I14 and I15. TEST CLK C is connected to a first input of NAND gateN25. EN is connected to first input of NAND gate N24 and OSC isconnected to a second input of NAND gate N24. The output of NAND gateN24 is connected to a second input of NAND gate N25 output of NAND gateN25 is connected to a first input of NAND gate N26 through inverter I13and to a first input of NAND gate N27. TEST CLK B is connected to asecond input of NAND gate N27. The output of NAND gate N26 is connectedthrough inverter I14 to ZC and the output of NAND gate N27 is connectedthrough inverter I15 to ZB.

Interconnections between output stage CS2 and latch stages Q7, Q8 and Q9are as follows: TEST is connected to a third input of NAND gate N31, athird input of NAND gate N34, a first input of NAND gate N37, a secondinput of NAND gate N38 and a first input of NAND gate N39. The output ofNAND gate N24 is also connected to a second input of NAND gate N29, asecond input of NAND gate N30 and the input of inverter I18. Node Q7B isalso connected to the input of inverter I17 (and thus the second inputof NAND gate N33). The output of node I15 is connected to a second inputof NAND gate N39. Node Q8B is also connected to a second input of NANDgate N37 and a first input of NAND gate N28. Node Q9B is also connectedto a second input of NAND gate N28. The output of NAND gate N37 isconnected to a second input of NAND gate N26 and a second input of NANDgate N27.

Compact splitter 195 operates in a similar manner as compact splitter190 of FIG. 1, except only two ZB pulses are generated. Latches Q7 andQ8 are master/slave “D” type latches. In order to reduce transistorcount, each master/slave latch could be replaced with a single latchconnected to a first input of an AND gate and also connected through aninverter to a second input of the same AND gate to provide a controlpulse sensitive to the rising edge of the corresponding TEST CLK B orTEST CLK C. However, the ZB and ZC pulse duration is a “designed in”delay and not a delay governed by rising OSC edges.

FIG. 16 is a timing diagram of compact clock splitter 195 of FIG. 15.The only difference between the timing diagram of FIG. 14 and that ofFIG. 16 is there are only two ZB pulses in the timing diagram of FIG.16.

Thus, the present invention provides a method and circuit for testingintegrated circuits at functional.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.For example, throughout the description of the present invention supra,types of latches called an L1L2 or MUX scan latches were used. The useof an L1L2 latch or MUX scan latch should be considered exemplary andother types of latches well known in the art may be substituted andadjustments to the test controller of the present invention made if andas required. Therefore, it is intended that the following claims coverall such modifications and changes as fall within the true spirit andscope of the invention.

1. A control circuit, comprising: a first latch having first, second,third and fourth inputs and first and second outputs; a second latchhaving first, second, third and fourth inputs and first and secondoutputs; a third latch having first, second and third inputs and anoutput; combinational logic having a test signal input, a test clockinput, a functional clock signal input, a first control signal outputand a second control signal output; said first output of said firstlatch connected to said first input of said second latch, said firstoutput of said second latch connected to said first input of said thirdlatch, said second output of said second latch connected to said firstinput of said first latch, and said output of said third latch connectedto said third inputs of said first latch and second latches; saidcombinational logic connected to said first, second and fourth inputsand said second output of said first latch, to said second and fourthinputs and said second output of said second latch and to said secondand third inputs of said third latch; wherein said combinational logiccouples said second output of said first latch to said second controlsignal output, couples said second output of said second latch to saidfirst control signal output, couples said second input of said firstlatch, said second input of said second latch and said second input ofsaid third latch to said functional clock signal input, and couples saidfourth input of said first latch, said fourth input of said second latchand said third input of said third latch to said test signal input; andwherein said combinational logic couples said test clock input, saidtest signal input and said second output of said first latch to saidsecond control signal output, and couples said test clock input and saidsecond out put of said second latch to said first control signal output.2. The control circuit of claim 1, wherein: said second latch isresponsive to start a first control signal on said first control signaloutput after the start of a second control signal based on a first stateof a test signal on said test signal input; said first latch isresponsive to start said second control signal on said second controlsignal output based on said first state of said test signal; said thirdlatch responsive to terminate said first control signal after a firstcycle of a functional clock signal applied to said functional clockinput and to terminate said second control signal after a second andconsecutive cycle of said functional clock signal after said first cycleof said functional clock signal based on said first state of a testsignal on said test signal input; and said combinational logicresponsive to decouple said first control signal from said first controlsignal output and to couple said test clock input to said second controlsignal output based on a second state of said test signal.
 3. Thecontrol circuit of claim 2, wherein said functional clock signal has ahigher frequency than a test clock signal applied to said test clockinput.
 4. The control circuit of claim 2, wherein said first and secondcontrol signals start and terminate in response to different edges ofsaid functional clock signal.
 5. The control circuit of claim 2,wherein: said second control signal starts a half a functional clocksignal cycle before said first control signal starts; and said secondcontrol signal terminates a half a functional clock cycle after saidfirst control signal terminates.
 6. The control circuit of claim 2,wherein edges of said first and second control signals are in phase withcorresponding edges of said functional clock signal.
 7. A method ofgenerating control signals, comprising: providing a first latch havingfirst, second, third and fourth inputs and first and second outputs;providing a second latch having first, second and third inputs and firstand second outputs; providing a third latch having first, second andthird inputs and an output; providing combinational logic having a testsignal input, a test clock input, a functional clock signal input, afirst control signal output and a second control signal output;connecting said first output of said first latch to said first input ofsaid second latch, connecting; connecting said first output of saidsecond latch to said first input of said third latch; connecting saidsecond output of said second latch to said first input of said firstlatch; connecting said output of said third latch to said third inputsof said first and said second latches; connecting said combinationallogic to said first, second and fourth inputs and said second output ofsaid first latch, to said second and fourth inputs and said secondoutput of said second latch and to said second and third inputs of saidthird latch; coupling said second output of said first latch to saidsecond control signal output; coupling said second output of said secondlatch to said first control signal output; coupling said second input ofsaid first latch, said second input of said second latch and said secondinput of said third latch to said functional clock signal input;coupling said fourth input of said first latch, said fourth input ofsaid second latch and said third input of said third latch to said testsignal input; coupling said test clock input, said test signal input andsaid second output of said first latch to said second control signaloutput; and coupling said test clock input and said second out put ofsaid second latch to said first control signal output.
 8. The method ofclaim 7, further including: said second latch starting a first controlsignal on said first control signal output after the start of a secondcontrol signal in response to a first state of a test signal on saidtest signal input; said first latch starting said second control signalon said second control signal output in response to said first state ofsaid test signal; said third latch terminating said first control signalafter a first functional clock signal cycle and terminating said secondcontrol signal after a second and consecutive functional clock signalcycle after said first functional clock signal cycle in response to afirst state of a test signal on said test signal input; and saidcombinational logic decoupling said first control signal from said firstcontrol signal output and coupling said test clock input to said secondcontrol signal output in response to a second state of said test signal.9. The method of claim 8, wherein said functional clock signal has ahigher frequency than a test clock signal applied to said test clockinput.
 10. The method of claim 8, further including: starting andterminating said first and second control signals in response todifferent edges of said functional clock signal.
 11. The method of claim8, further including: starting said second control signal a half afunctional clock signal cycle of said functional clock signal beforestarting said first control signal; and terminating said second controlsignal a half a functional clock cycle of said functional clock signalafter terminating said first control signal.
 12. The method of claim 8,wherein edges of said first and second control signals are in phase withcorresponding edges of said functional clock signal.
 13. A controlcircuit, comprising: a first latch having first, second, third andfourth inputs and an output; a second latch having first, second, third,fourth and fifth inputs and first and second outputs; a third latchhaving first, second and third inputs and an output; combinational logichaving a test signal input, a functional clock signal input and a scancontrol signal output; said first output of said first latch connectedto said first input of said second latch, said first output of saidsecond latch connected to said first input of said third latch, saidsecond output of said second latch connected to said first input of saidfirst latch, and said output of said third latch connected to said thirdinputs of said first and second latches; said combinational logicconnected to said first and fourth inputs of said first latch, to saidsecond, fourth and fifth inputs and said first output of said secondlatch and to said second and third inputs of said third latch; whereinsaid combinational logic couples said first output of said second latchto said scan control signal output, couples said second input of saidfirst latch, said second input of said second latch and said secondinput of said third latch to said functional clock signal input, andcouples said fourth input of said first latch, said fourth input of saidsecond latch and said third input of said third latch to said testsignal input; and wherein said combinational logic couples said testsignal input, said scan control input and said first output of saidsecond latch to said scan control signal output.
 14. The control circuitof claim 13, wherein: said first latch is responsive to generate atrigger signal coupled to said second latch based on a first state of atest signal on said test signal input; said second latch is responsiveto said trigger signal to generate a starting edge and a terminatingedge of a scan control pulse based on said first state of said testsignal; said third latch is responsive to prevent starting anotherstarting edge of another scan control pulse until said test signalreturns to said first state from a second state; and said combinationallogic is responsive to decouple said first output of said second latchfrom said scan control signal output and to couple said scan enableinput to said scan control output based on a second state of said testsignal.
 15. The control circuit of claim 14, wherein said functionalclock signal has a higher frequency than a scan control signal on saidscan control input.
 16. The control circuit of claim 14, wherein saidstarting and terminating edges of said scan control pulse are generatedin response to edges of said functional clock signal.
 17. The controlcircuit of claim 14, wherein said scan control pulse has a duration of asingle cycle of said functional clock cycle in duration.
 18. The controlcircuit of claim 14, wherein said starting and terminating edges saidscan control pulse are in phase with corresponding edges of saidfunctional clock signal.
 19. A method of generating a control signal,comprising: providing a first latch having first, second, third andfourth inputs and an output; providing a second latch having first,second, third, fourth and fifth inputs and first and second outputs;providing a third latch having first, second and third inputs and anoutput; providing combinational logic having a test signal input, a scancontrol input, a functional clock signal input and a scan control signaloutput; connecting said first output of said first latch to said firstinput of said second latch; connecting said first output of said secondlatch to said first input of said third latch; connecting said secondoutput of said second latch to said first input of said first latch;connecting said output of said third latch to said third inputs of saidfirst and second latches; connecting said combinational logic to saidfirst and fourth inputs of said first latch, to said second, fourth andfifth inputs and said first output of said second latch and to saidsecond and third inputs of said third latch; coupling said first outputof said second latch to said scan control signal output; coupling saidsecond input of said first latch, said second input of said second latchand said second input of said third latch to said functional clocksignal input; coupling said fourth input of said first latch, saidfourth input of said second latch and said third input of said thirdlatch to said test signal input; and coupling said test signal input,said scan control input and said first output of said second latch tosaid scan control signal output.
 20. The method of claim 19, furtherincluding: said first latch generating a trigger signal coupled to saidsecond latch in response to a first state of a test signal on said testsignal input; said second latch, in responsive to said trigger signal,generating a starting edge and a terminating edge of a scan controlpulse based on said first state of said test signal; said third latchpreventing starting another starting edge of another scan control pulseuntil said test signal returns to said first state from a second state;and said combinational logic decoupling said first output of said secondlatch from said scan control signal output and coupling said scan enableinput to said scan control output in response to a second state of saidtest signal.
 21. The method of claim 20, wherein said functional clocksignal has a higher frequency than a scan control signal on said scancontrol input.
 22. The method of claim 20, further including: generatingsaid starting and terminating edges of said scan control pulse inresponse to edges of said functional clock signal.
 23. The method ofclaim 20, wherein said scan control pulse has a duration of a singlecycle of said functional clock cycle in duration.
 24. The method ofclaim 20, wherein said starting and terminating edges said scan controlpulse are in phase with corresponding edges of said functional clocksignal.